The inlA gene required for cell invasion is conserved and specific to Listeria monocytogenes

The Gram-positive bacterium Listeria monocytogenes can actively induce its own uptake by epithelial cells and fibroblasts through a surface-exposed 80 kDa protein, internalin (InlA), encoded by inlA. We studied the distribution and the DNA polymorphism of inlA sequences in a wide variety of wild strains of L. monocytogenes as compared to other Listeria species. This was done by PCR-amplifying inlA sequences encoding the fifteen repeats A and the three repeats B of InlA. inlA-repeated sequences were only found in L. monocytogenes. The amplified fragment of inlA encoding the repeats A displayed an Alul DNA polymorphism which arises from point mutations. These results indicate that inlA required for cell invasion is specific to L. monocytogenes and that the intragenic repeats only exhibit a genetic heterogeneity due to point mutations and not to recombinations.